The present invention relates generally to detection circuits and, more particularly, to an improved detection circuit for conveyor belt rip detectors. The invention also is directed to a circuit for converting a pulse height modulated AC signal to a DC pulse and to a variable level detecting circuit, with these being described in detail below with reference to their inclusion in the detection circuit of a conveyor belt rip detector system.
In large endless conveyor belts of the type having a non-metallic body used to convey bulk material, there is a possibility of encountering a rip in the belt caused, for example, by a sharp object dropped thereon at the loading station. It is desirable promptly to detect such rips and, preferably, to shut down the conveyor belt upon such detection, thereby minimizing damage to the belt.
In. U.S. Pat. No. 3,792,459 there is disclosed a conveyor belt rip detector of the type with which the detection circuitry of the present invention may be employed. In such rip detector a plurality of antennae, which may be single electrical conductors, are embedded transversely at spaced-apart locations in the belt so as periodically to couple an electrical signal from a transmitter to a receiver while the belt moves and the respective antennae pass respective capacitive connections to the transmitter and receiver.
The electrical signal charge capacitively induced in each of the unbroken antennae is, thus, transmitted through to the receiver and is sensed by the detector circuitry thereof. However, a broken antenna, for example at a place where the belt has been ripped, will not couple the transmitter signal through to the receiver, and the detector circuit, then, senses the same. Moreover, the detector circuit disclosed in such patent is operative after the lapse of a predetermined time period, which corresponds to the time required for passage of a given number of broken antennae past the transmitter and receiver without the coupling of the transmitter signal to the receiver, to produce a distinguishable output that activates an alarm and/or deactivates the conveyor belt drive.
In such a rip detector system the raw input signal received at the receiver from respective antennae passing the same is an AC pulse height modulated signal, the AC portion being derived from the transmitter and the pulse height modulation occurring in response to the proximity of the respective antennae to capacitive connection plates associated with the transmitter and receiver. The pulse height of such modulated signal, therefore, provides the characteristic information indicative of the integrity condition of the belt. Thus, the non-presence of a pulse, when one is expected, or the presence of pulses with too small a pulse height are interpreted by the detector circuitry as a fault or ripped belt condition. However, conveyor belts and rip detectors often are used in electrically noisy environments, and such electrical noise may incorrectly be interpreted by the detector circuitry as the passage of an antenna. Moreover, the spacing of the capacitive coupling plates or the like relative to the conveyor belt and the antennae may vary, for example, as the conveyor belt stretches and shrinks with use, temperature, etc., thereby creating corresponding variations in the magnitude of the pulse received by the receiver.
It is, accordingly, desirable to avoid the detriment effected by such electrical noise and/or spacing variations, thereby to improve the reliable operation of a conveyor belt rip detector system.